Adjustable optical lens systems comprising fluids are known from the prior art.
WO07049058 for example, was published in May 2007 and is directed to a lens with a variable focus, which comprises a rigid ring to which a flexible membrane is attached. A rigid transparent front cover is attached to the flexible membrane and a rigid rear cover on the rear surface of the ring. A cavity is formed between the flexible membrane and the rear cover which is filled with a liquid. The amount of liquid in the cavity can be adjusted to vary the curvature of the flexible membrane and so vary the optical characteristics of the lens. A second flexible membrane can be positioned between the rear cover and the ring.
US2006164731AA was published in July 2006 and shows another example of a lens with a variable focus which is based on a liquid-filled chamber (cell). The liquid lens cell consists of four parts, namely a clear distensible membrane, a transparent wall membrane, a liquid with a fixed volume stored in the lens cell chamber and an annular periphery sealing ring. The radius of the annular sealing ring is changeable, similar to a conventional iris diaphragm. By tuning the radius of the annular sealing ring, the stored liquid in the lens cell will be redistributed and thereby change the curvature of the distensible membrane. One disadvantage consists in that the liquid lens is influenced by gravity forces and vibrations.
WO06011937 was published in February 2006 and is directed to a fluidic adaptive lens device with at least one flexible membrane (indicated as first partition). The adaptive lens includes a first flexible and optically transparent membrane. A second partition, which is coupled to the flexible membrane, is at least partially optically transparent. A first cavity is formed in between the flexible membrane and the second partition. The lens device comprises a fluid within the cavity. Furthermore the device comprises means, e.g. a teflon coated screw, to control the pressure or the volume of the fluid in the chamber. When the parameter of the fluidic medium changes, the membrane flexes and the optical property of the lens changes.
US2003095336 was published in May 2003 and describes a lens arrangement mainly for a corrective or a prescription lens. The prescription lens is adjacent to a fluid cell which has a flexible membrane and a base. In that fluid is pumped into or out of the fluid cell the corrective power of the entire lens arrangement is varied. The device is only limitedly suitable for miniaturisation.
U.S. Pat. No. 5,446,591 was published in August 1995 and discloses a further adjustable lens assembly for a telescope objective. The device comprises a liquid lens element contained between adjacent glass lens elements. Liquid is introduced into the gaps between adjacent glass lens elements to form the liquid lens elements.
U.S. Pat. No. 4,890,903 was published in May 1987 and is directed to an adjustable liquid lens or mirror which has a chamber delimited by a flexible membrane. The flexible membrane is supported in tension between a pair of resilient rings. A first fluid medium fills the chamber which, in the case of a lens, has a different refractive index from that of a second fluid medium contacting the other side of the flexible membrane. An annular support member for the flexible membrane comprises relatively movable first and second component parts, these first and second component parts of the support member being adjustably linked in a fluid-tight manner, whereby the volume of the chamber is adjustable by moving one component part of the support member relative to the other in such a way as to vary the pressure in the first fluid medium and thereby to alter the shape of said membrane surface.
U.S. Pat. No. 0,154,380 A1 was published in October 2002 and discloses micro-machined devices, acting as electro-mechanically tuneable concave lenses. The lens-body itself is made of media including electrically conductive and transparent electrodes, as indium tin oxide, and a membrane separating two regions of differing refractive index. By applying a voltage over the electrodes, within the lens-body, an electrostatic force acts over the within certain ranges flexible electrodes, the lens-body in between is reduced in its thickness, and the optical properties are therefore changed.
JP 144975 A was published in May 1998 and describes a tuneable liquid-filled lens, using a ring-shaped piezo actuator. In the inner opening of the actuator, a transparent cover allows the light to pass trough, while the outer rim is fixed on an rigid ring with a certain depth. In the centre of the ring, in opposite direction of the piezo actuator, a flexible and transparent membrane encompasses with the actuator a liquid-filled space, and with applying a voltage on the actuator, the spanned volume changes. With changing the volume, the membrane deflects in one direction and builds a calotte-shaped, tuneable lens.
WO 096028 A was published in October 2005 and describes a tuneable, liquid-filled lens using a ring shaped piezo actor as well. Here, a laterally generated deflection of the piezo alters the diameter of a cylindrical liquid volume. The constant volume of the liquid itself creates a pressure on a neighbouring and immiscible liquids with same density but different refractive index. With applying a voltage on the piezo, the interface of the liquids changes its position, and a tuneable lens is generated.
GB 1327503 A was published in August 1973 and describes a piezo-driven, tuneable liquid-lens. A certain volume of liquid encompasses a closed cylindrical and piezoactive box, optically transparent in the axial direction. The liquid itself is encompassed by a rigid box, on one top closed by an elastic and trans-parent membrane. By applying a voltage on the piezo, the inner volume is changed, and the membrane deflected, and therefore a tuneable lens is generated.
U.S. Pat. No. 164,731 A1 was published in July 2006, and describes a tuneable liquid-filled lens, where the volume of a cylindrical chamber is changed by mechanically tuning the diameter of the wall. In the axial direction, the system is closed by flexible membranes, allowing to deflect while changing the diameter of the box. In that way, a tuneable lens easily can be generated.
WO08020356 was published in February 2008 and is directed to a variable focus lens. The lens comprises a container having an interior chamber. A first fluid medium and a second fluid medium are disposed in the chamber and are separated by a meniscus. A meniscus control system, for controlling a shape of the meniscus, comprises a first control element and a second control element. The first control element is coupled to the meniscus and is moveable in a direction substantially parallel to the optical axis. The first control element and the second control element are configured to interact using an electric field or a magnetic field. The interior chamber may be a closed chamber without any elements extending through the wall of the chamber. Hence, a chance of leakage of the fluid media from the chamber is reduced. One problem results from the sealing between the first control element and the side wall of the chamber.
One disadvantage of optical lens systems known from the prior art consists in that they have a complicated setup with means to exchange fluid such that relative internal pressure and volume can be influenced.
It is an object of the present invention to provide an improved liquid lens system having a simple and robust setup.
An embodiment of a lens system according to the present invention is, unlike the lens systems known from prior art, an in principle closed system with at least one main chamber arranged inside an outer housing. The at least one main chamber is at one side delimited by a flexible membrane with respect to at least one additional chamber or the outside. The at least one chamber is normally completely filled with a fluid (gas and/or liquid) having the same or different index of refraction then the fluid present in the adjacent area.
Under a closed system in the sense of the present invention a system is understood where no exchange of fluids with the outside (external exchange) is necessary during normal operation. However, if appropriate, a certain amount of fluid can be exchanged with the outside e.g. for adjustment reasons of the initial position or shape of the membrane. The at least two chambers and thereby the therein contained fluids are delimited with respect to each other at least partially by a flexible membrane. Depending on the field of application, the membrane can be designed as one continuous membrane or a membrane consisting out of sections having the same or different mechanical and optical properties. The shape, respectively the deflection of the membrane (barrier layer) between the fluids and thereby the optical properties of the system are adjustable via an actuator which is mechanically interconnected to or integrated in the at least one membrane. In special applications part of the membrane can be formed as rigid part e.g. if it becomes necessary to deflect the barrier layer in a parallel manner (e.g. in phase shifting applications). The actuator can be completely integrated into an outer housing of the lens system or arranged at least partially outside. Good results are obtained by actuators which act upon the membrane based on Coulomb forces or by magnetic actuators which act upon the membrane from the outside.
One embodiment of a lens system normally comprises on one side of a membrane several chambers or areas which are interconnected to each other e.g. via channels or openings to exchange volumes of fluid within the system and to thereby influence the optical characteristics of the lens system. In that the chambers inside the lens system are normally filled by a constant amount of fluid, gravity forces and thereto related local deformation can be compensated because the fluid pressure is in certain tolerances everywhere in the system the same. Unlike the embodiments known from prior art an optical system according to the invention is normally not position-dependent and gravity forces have not negative influence. In that the at least one actuator means to deform the volumes of fluid is integrated into the system, it is possible to avoid external reservoirs. In addition a sealed system offers the advantage that contamination can be avoided. In that the membrane separates the at least two chambers with respect to itself or the at least one chamber with respect to the surrounding, a simple and yet efficient construction is possible. No sealing problems as know from the prior art occur.
A lens system according to the invention has in general an outer housing with a central main opening extending axially through the housing. The opening can be closed at least on one side by a rigid or flexible panel made of optically active or transparent material as glass, plastics, elastomers or metals. If appropriate, several lens systems may be lined up next and optically interconnected to each other. Thereby it becomes possible to omit certain separating panels. The panel itself may be shaped as a lens or comprise diffractive refractive or reflective structures. Alternatively or in addition further lenses which can be variable or fixed focus may be foreseen to influence the light path.
In certain embodiments a CCD-array (or a similar device) may be integrated in the lens-system forming together a complete module. If appropriate the module incorporates electronic circuits to control the actuation and the focus of the system and/or to process picture information recorded by the CCD-array.
At least one membrane is arranged in the opening of the housing, if appropriate in a stretched and/or prestretched manner under tension, extending across the opening and thereby separating the opening in axial direction in two opposite chambers. The membrane contains at least two regions of in general antiparallel deflection during actuation, and may be prestretched and/or the shape may be determined by the relative amount of liquids filled into the chambers. Depending on the field of application more than one membrane may be present. At its outer end the membrane is normally fixed to the outer housing. As will be explained subsequently in more detail, the membrane may be stretched and fixed to additional holding means (annular holding frames) which are arranged inside the opening to delimit certain areas. Depending on the field of application, the membrane is arranged at least partially in an non-planar way.
The membrane is normally attached to additional holding means whereby areas of the membrane with higher tension/strain may be adjacent to areas of the membrane with lower tension/strain. In a process for the making of a lens system this can be achieved in that a membrane is stretched to a first extent, then fixed to e.g. a frame-like holding means. The area of the membrane surrounding the holding means is then stretched to a second extent. Stretching may take place by mechanical means or thermal means (e.g. by hot gas or radiation). Alternatively or in addition the initial position of the membrane may be determined by filling the at least one chamber with a fluid.
In an embodiment of the lens system, a ring-shaped holding frame which acts as holding means is arranged inside the opening of the housing to which the membrane is attached in a concentric manner. The membrane is attached to the ring-shaped holding frame. Depending on the field of application, the area of the membrane arranged inside the holding frame is normally less stretched than the outer area of the membrane arranged between the holding frame and the housing. The area of the membrane inside the holding frame is optically active and adjustable with respect to its optical characteristics. The optical characteristics of the lens system are adjusted by an actuator which is interconnected directly or indirectly to the optical active part of the membrane. In that the optically active element is subjected to less strain than the outer area of the membrane, the outer area of the membrane and the axial position of the annular holding means dominates the geometry of the optical active inner part of the membrane via the displacement of fluid. In an embodiment an actuator acts directly upon an annular outer part of the membrane surrounding the holding frame causing a deflection of said part of the membrane. The deflection of the optical active part of the membrane arranged inside the holding frame is caused indirectly by the fluid arranged in the chambers on both sides and thereby mechanically coupling the membranes. In that part of the fluid is displaced by the actuated movement of the outer part of the membrane, the position of the holding means or combinations thereof, the optical active inner membrane is deflected as a result thereof.
In an embodiment the optical active part of the membrane has a calotte shape with a certain radius. This is achieved by the different strain (resp. stress) in the different sections of the membrane and the relative amount of fluid filled in the chambers. In that the actuator changes the relative strain in the different sections of the membrane fluid is moved in the at least one chamber and due to the tendency of the membrane material to contract, the shape of the calotte is altered, e.g. by decreasing or increasing the radius. By the thickness distribution of the membrane, it is possible to influence the shape of the calotte, to e.g. parabolic or another appropriate design. Thereby it becomes possible to correct optical errors.
Good results are achieved by actuators in form of two electrodes in general arranged opposite to each other on either side of the membrane or a section thereof. Alternatively or in addition the membrane can be covered by a magnetic layer or be made out of a magnetic layer itself such that the membrane can be deflected by a magnetic field. At least one electric coil arranged inside or outside of the housing suit to deflect the membrane. The actuator encompasses the elastically deformable and if appropriate prestretched membrane, e.g. consisting of elastomeric material. Depending on the embodiment, the electrodes are arranged sufficiently electrically isolated with respect to each other to avoid negative flow of current.
In the case that the membrane is covered by two opposite electrodes, by applying a voltage between the first and the second electrode, the intermediate layer in the area between the first and the second electrode is compressed by coulomb forces, respectively Maxwell Stress causing a local reduction of thickness (first direction) of the prestretched membrane material. The poisson's ratio of the membrane material causes a lateral, in-plane expansion of the membrane (secondary deformation). If appropriate, further deformations may be superimposed. The lateral expansion causes an out-of-plane deflection of the normally stretched membrane such that the characteristics of the optically active membrane changes in a determined manner.
To obtain special optical effects the membrane can be three dimensionally shaped or have a variable thickness or contain diffractive, refractive, reflective scattering or absorbing structures. Alternatively or in addition other optical functions such as phase shifting functions, tuneable micro-lens arrays or tuneable mirrors may be implemented. If appropriate the membrane can be made section wise out of different materials or layers. Good results are obtained in that the membrane consists out of commercially available VHB4910, VHB 4905, VHB 9460 tape of 3M. This material has a refractive index in the range of 1.47. Good results are also obtained, using elastic membranes containing silicon, silicon gels or urethanes.
A device to correct lens errors can be obtained in that a membrane is arranged inside of a housing separating the housing into a first and a second chamber which are filled by liquids having in general a similar index of refraction and the membrane is made out of a material having a different index of refraction. In that the membrane is deflected as described above, e.g. such that it forms a calotte, it is achieved that the optically active part of the membrane has in the centre a lower inclination which increases with the distance to the centre (in radial direction). The light passing through the lens system faces a longer path in the outer area then in the centre. Thereby it is e.g. possible to compensate an optical error of an interconnected lens or another optical device. To optimize the effect, the membrane may contain a thickness distribution, acting as a flexible lens itself. Depending on the setup such as the design of the membrane it is possible to compensate aberrations such as spherical or chromatic aberration.
A phase shifting device can be obtained in that an elastic membrane as described above comprises an non-deformable planar centre part which is deflectable by an actuator in axial direction (along the path of the light). The device comprises at least two chambers which are filled by fluids having different index of refraction. In that the planar centre part is deflected it is achieved that the path of the light in the different media changes which results in a phase shifting effect. The planar centre part can be made out of a transparent or a reflective material.
An adaptive microlens array can be obtained by arranging a rigid, and non-deformable centre part, containing an arrangement of small openings, which are covered by an elastic, and deformable layer of optically transparent material. By changing the volume of fluid within the inner chamber, the shape of the deformable layer over the openings is altered, and the optical properties of the device is affected.
In that light-absorbing fluids is filled in the at least one chamber, the intensity distribution of a light beam can be affected. Here, as a function of the lateral position, the optical path within the absorbing fluid is varied and therefore the total absorption on a specific optical path within the inner chamber is controlled.
In a lens system according to the present invention the membrane can be coated with an antireflection layer. The at least one membrane can have a multi-layered setup whereby the index of refraction of the individual layers as well as the thickness of the layers may be adjusted such that reflected beams are eliminated by destructive interference.
A coatings may be applied in different manners to the surface of the membrane and/or the housing. Good results are obtained by conventional methods such as vapor deposition, plasma coating, doping, self assembled monolayers (SAM's), Langmuir Blodgett Films, amphiphile Surfactants or spin coating. A method is the application of certain 3D structures on the membrane itself. By structuring the surface with adequately distributed pimples or objects in sub-wavelength range, an antireflection effect can be generated. Methods to generate the effect is etching the membrane, casting or application of particles on the membrane.
If appropriate the molecules of the coating may have a certain solubility in the fluid present in one of the adjacent chambers and a sedimentation may occur onto the surface of the membrane. The similar effect can be achieved in that a highly viscose liquid layer with a high chemical affinity to the material of the elastic material and with a low solubility in the surrounding liquid (fluid) is applied to the surface of the membrane. E.g. a layer of oil is applied onto a lipophilic surface of a membrane.
A lens system according to the invention may be used in applications, where the compact control- and steering mechanism for changing focal distances is of interest, as in hand held devices, such as cellular phones or personal digital assistants, projectors, cameras, objectives for optical measurements, high power laser control applications, interferometers, displays or microscopes. Using appropriate materials, e.g. biocompatible materials, it can be used for medical applications or implantation in mammalians, for corrections of visual faculty. In contrast to the prior art, one advantage consist in that a lens system according to the invention can easily be miniaturized or scaled in size.
The fluid arranged in the chambers is preferably out of the group of silicone oil, oils, solvents, water, transparent or reflective liquids, gas. If appropriate it is possible to substitute the fluid at least partially by a gel such as e.g. Silgel 612 A&B of Wacker, or Sylgard 527 or Sylgard 528 of Dow Corning. Gel has the advantage that sealing is less a problem. Preferably at least one of the fluids is of an incompressible type, such as liquids, e.g. oil or water. For certain applications, the fluid may contain particles or objects to affect the optical behaviour. Such a dispersion can be used to strongly affect the propagation and spreading of electromagnetic waves which differ from visual range. The lens system therefore can be used for different electromagnetic wave spectra.
In that one chamber of the lens system is filled by a reflective material such as mercury, it is possible to make a mirror type of lens system. Alternatively or in addition the membrane itself can be coated by thin, flexible or liquid metals as alloys made of Gallium, Indium and Tin as “Galinstan” or other eutectic alloys or consist of a reflective material itself. The same effect can be obtained by simply placing a mirror on one side of the optical active part.
If appropriate the membrane can be made out of a semi-permeable material which allows the passage of gas inside a liquid arranged in the at least one chamber. Due to the higher internal pressure of the liquid in the chamber the gas is forced out of the chamber through the membrane. Gas bubbles having a negative effect can thereby be omitted.
If appropriate, e.g. in beamers or high performance spot lights, the lens system can be cooled by a closed cooling circuit and/or by convective type of cooling. The cooling circuit is preferably isobaric to not influence the position of the membrane in an unintentional manner.
A process for making a lens system according to the present invention with a prestretched membrane in general comprises the following process steps:                1. Stretch a membrane material to a first extent;        2. Attach the stretched membrane material to an holding frame, resp. an annular holding frame;        3. Stretch the membrane surrounding of the holding to a second extent;        4. Arrange the membrane in an opening of a housing and attach the membrane to the housing;        5. Depending on the embodiment, attach opposite electrodes and/or at least one magnetic layer to a section of the membrane;        6. Fill a chamber delimited by a first side of the membrane with a first fluid having a first refractive index such that the part of the membrane stretched to a first extent forms a calotte;        7. Depending on the embodiment, fill a second chamber delimited by a second side of the membrane with a second fluid having a different refractive index.        
If appropriate the membrane is not arranged in a prestretched manner during making but is brought into a stretched shape by filling an appropriate amount of fluid into the chambers. The initial position may be determined by the relative amount of liquid filled into the chambers adjacent to the membrane.
If appropriate, the liquid can be filled in the chambers, before fixing the membrane holders. In that way, the final shape is generated during the assembly process itself. In that the material of the membrane is made out of a semi-permeable material, trapped air bubbles can diffuse through the membrane.
The application of a vacuum enables a faster degassing. This production method enables so-called wafer-level processes.
An embodiment of the invention is directed to an optical system comprising a housing with an opening extending in axial direction. At least one membrane is arranged across the opening, defining at the inside of the housing at least one chamber filled with an in general constant amount of a fluid. The membrane comprises an optically active and an optically passive section and at least one actuator to influence the geometry of the optically active section of the membrane preferably by relocation of the fluid, thereby changing the optical characteristics of the optical system. The optically active and the optically passive sections of the membrane are normally attached to at least one annular holding frame. In preferred embodiments the optically active and the optically passive sections of the membrane are attached to the same annular holding frame. The annular holding frame separates the membrane in an optically active and an optically passive section. In an embodiment the actuator is interconnected to the optically passive section of the membrane. Alternatively or in addition an actuator is interconnected to the annular holding frame to displace the annular holding frame in axial direction. The annular holding frame may be arranged at a certain distance to an inner side surface of the opening. In an embodiment the actuator to displace the membrane consists out of at least two electrodes which are interconnected to the membrane, electrically isolated with respect to each other and encompassing at least partially one section of the membrane. Good results are obtained if the electrodes are made out of metallic powders, conductive eutectic alloys, carbon black or an optically transparent electrode material. In an embodiment the at least one membrane separates the inside of the housing into a first and a second chamber filled with a first and a second fluid having the same or a different index of refraction. If appropriate, the optically active part of the membrane can contain rigid, absorbing, refractive, diffractive, diffusive or reflective structures. Depending on the field of application, the membrane is arranged perpendicular to the axis of the opening or at a certain angle to it.